C18Q1 Learn electronics and build a pedal

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  • m_cm_c Frets: 1211
    Vref is a reference voltage.
    As you've worked out, it is a result of the resistor pair, giving just a touch under 5V.

    It is then used via various other resistors/diodes/capacitors, to feed the inputs to the opamps, and ensure that all signal processing happens at positive voltages, with the centre being the reference voltage. To fully understand it you need to understand each step of the process, and the signal at that step.

    So starting with the input, you have an AC signal, that centres around GND (0V). The first thing it hits is a resistor and a coupling capacitor. The resistor is what defines your input impedance. It handles a mix of ensuring there is some load on the signal, reduces noise, avoids static build up, and couples the AC signal to GND.
    Now with an AC signal coupled to 0V, it means it's continually moving between positive and negative voltages, and handling both positive and negative voltages would introduce a lot of extra complexity into the circuit. So what the coupling capacitor does, is instead of the AC signal being coupled to 0V, it couples it to your reference voltage via the 510K resistor, so instead of the signal moving between say -0.5 to 0.5V, it's now moving between 4.5V and 5.5V (assuming Vref is 5V, and your capacitor is perfectly coupling the signal - which it won't!)
    This then means that you can do whatever signal processing/manipulation you want using only positive voltages.
    However, because you're now using the reference voltage as your signal centre point, everything has to work from that reference voltage.
    And then finally, before leaving the pedal, the signal passes through another coupling signal, where the signal is then coupled back to GND.

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  • Danny1969Danny1969 Frets: 10356
    edited January 2018
    Philtre said:
    Danny1969 said:
    Philtre said:
    What is a "Vref" and why is it needed? Is it a positive or negative supply? From my (limited) knowledge the two resistors act as a voltage divider to create a smaller voltage (5v I think in this case).
    Opamps are designed to run from a duel supply, like +15 - 0 and -15V  ... a pedal only has a single +9V to run from so  basically by biasing the non inverting input halfway of the 9V we can pretend +4.5 V is 0V and then block the DC on the output with a cap
    OK, but the opamp still needs a +9v supply to pin 8 and negative ground to pin 4, right?
    Yes, if you imagine we connect 8 to +9V and 4 to ground THEN we bias pin 3 at half the supply of 4.5V. Now our guitar signal which is an AC voltage can either swing our opamp upwards from 4.5V to near the 9V or negative from 4.5V towards ground. The only downside is we now need to block the permanent 4.5V on the input and output of our circuit from interfering with other devices in our chain, so we use caps on the input and output which block the 4.5V DC but allow the AC signal element to pass through
    www.2020studios.co.uk 
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  • PhiltrePhiltre Frets: 4164
    edited January 2018
    Great explanation @m_c and @Danny1969 - thanks! Wizs all round!
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  • PhiltrePhiltre Frets: 4164
    edited January 2018
    Quick progress update.

    All the components have arrived from Bitsbox and I've made a start assembling them onto the breadboard. Progress is a bit slow because of the following:
    • My eyesight isn't as good as it used to be and these components are bloody tiny and fiddly
    • Figuring out the best way to lay out the components on the breadboard
    • The breadboard I have is cheap, and some of the contacts are are a bit dodgy and stiff

    I also learnt about why a diode is used for polarity protection, and when to use a diode in parallel to the voltage supply, when to use a Schottky diode in series, about forward voltage drops, and how a bridge rectifier works (that's bloody clever that is). Woo, get me... ;-)

    Oh yes, I also learnt about the importance of putting a "load" across your 9v battery when taking multimeter readings. That diode got bloody hot when strapped directly across the battery terminals. ;-)

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  • Adam_MDAdam_MD Frets: 3420
    Philtre said:
    • My eyesight isn't as good as it used to be and these components are bloody tiny and fiddly
    Get a magnifying lamp I have a daylight one which has been one of the best pedal building purchases I've made. I used to get really bad headaches if I'd spent a long time working on a pedal but that stopped once I grabbed a lamp with decent magnification.  

    It's been useful for all sorts of stuff outside of pedal building too.  
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  • PhiltrePhiltre Frets: 4164
    Adam_MD said:

    Get a magnifying lamp I have a daylight one which has been one of the best pedal building purchases I've made. I used to get really bad headaches if I'd spent a long time working on a pedal but that stopped once I grabbed a lamp with decent magnification.  

    It's been useful for all sorts of stuff outside of pedal building too.  
    I tried a round magnifier once but I couldn't handle the the curvature of the glass, things looked distorted. Can you get flat ones?

    Oh, and can anyone recommend good wire strippers for single core 22 AWG wire (breadboard wire)? I'm currently using a cutting knife and my fingernails, and it's doing my head in. ;-)
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  • m_cm_c Frets: 1211
    For solid core wire, the best option is manually adjustable wire strippers. The type with two V-shaped jaws, with a screw to set how close the V's get, as that way you're cutting the insulation at 4 points.

    Automatic strippers, that only cut on two sides, aren't any better than a pair of side cutters or Stanley knife, where solid core is involved. Off course it depends on the exact insulation, as some will strip easier than others.
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  • PhiltrePhiltre Frets: 4164
    edited January 2018
    Success! And it worked first time. :-)

    The breadboard layout is clearly not the best, but this is really the first time I've done this for a proper circuit, and, hey, it works.

    It's the Timmy circuit above, and it doesn't sound too bad. :-)

    Image1


    Lessons learnt:
    • Trim pots save space, but it's a bugger if you want to play with the knobs
    • Ditto for the DIP switch - difficult to access
    • I need to come up with a better solution for offboard sockets and pots
    • Breadboarding can potentially lead to more errors as the connections can be sketchy

    So, for me, this was a successful proof of concept exercise. I've proved that I can get it together to buy and assemble the components, interpret a schematic and make the thing work.

    And now I need to think about how to make this permanent. Options:

    1. Veroboard it together and re-use my components and buy pots, switches and an enclosure
    2. As option (1) but locate a PCB. But this might use onboard pots and I'd need to be careful with drill holes
    3. Buy the whole goddam kit and solder it together (like this one)
    And for being such a clever dick and putting together my very first breadboard pedal I'm going to drink a lot of wine. ;-)
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  • Danny1969Danny1969 Frets: 10356
    Well done @Philtre ..... that's good progress and a lot learned

    Rather than put jack sockets on the breadboard it's  easier to have a couple of jack sockets with wires soldered to them so you can make contact with the breadboard circuit easily .... likewise with pots 

    Personally I tend to build on veroboard from the off with every joint soldered. It's less prone to bad connections and you very quickly learn how to build compact sensible layouts. 

    For prototyping I use a lot of salvaged electronics from all kinds of broken radios, speakers, mains adapters ... if you get some braid a decent solder sucker you can harvest pretty much any through hole component easily and it;s not that hard to do SM

    Here's a little vid I made showing the build of a Tubescreamer by harvesting the opamp and passive compnents from an old computer speaker and a tablet power supply 



    One of the cheapest options for wire is 6 core alarm cable. When you remove the sheaf you have 6 colours of wire and I like to stick to a coloured system that makes fault finding easier like this :-1: 

    Red : + Volts
    Black : - Volts 
    Green : Ground
    White : signal in 
    Blue : signal out
    Yellow : signal control function 

    Your gonna pay more buying from Bitsbox, some of the components are 4 times the price I pay for common pots. opamps and caps 

    Happy building in 2018 and keep reporting the progress 


    www.2020studios.co.uk 
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  • PhiltrePhiltre Frets: 4164
    Thanks for sharing @Danny1969 - very cool. ;-) Nice playing (do you like Jeff Beck?)
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  • PhiltrePhiltre Frets: 4164
    I've been playing my new Timmy Clone prototype with my Jazzmaster into a HRD all night. I reckon it's my favourite pedal right now. ;-)

    Right, I want to get this built onto a proper PCB or Vero and into an enclosure... :-)

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  • Danny1969Danny1969 Frets: 10356
    Mmm your making me want to build a Timmy now too ... it is supposed to be pretty good and I'm using an HRD too 

    Looking at the circuit Veroboard be fine, not complicated enough to warrant a PCB 
    www.2020studios.co.uk 
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  • Adam_MDAdam_MD Frets: 3420
    The Timmy pedal works really well with a HRD.  
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  • PhiltrePhiltre Frets: 4164
    I had intended to try out different chips, but the breadboard seems so precarious I'll wait until it's on a vero board and use a socket on it and try then.

    Quick question to the pedal builders - if I use a vero board how do I fix it in the enclosure, or can it just float around inside?
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  • Adam_MDAdam_MD Frets: 3420
    I just looked at the picture properly did you order those caps yourself or as part of the Timmy kit @Philtre?  I’m assuming you ordered them rather than them coming in a kit as you’d never get some of those in a 1590b.  
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  • Adam_MDAdam_MD Frets: 3420
    edited January 2018
    Keep the cables to the pots pretty short and tidy and they’ll hold it in place.  

    I was trying to post a picture but imgur isn’t working tonight either the site or the app.  The second pedal down on this post is on vero and the board is being held in place by the cables to the pots.  Vero works it’s fun and is the ultimate in DIY but for neatness I do prefer a pcb and board mounted pots.

    http://thefretboard.co.uk/discussion/114178/getting-back-into-building-new-update#latest
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  • PhiltrePhiltre Frets: 4164
    @Adam_MD I got the IC and some caps from Bitsbox. I already had the 1N4148s and the 47uF electrolytics. The big caps are 1uF polyester films from Bitsbox, I couldn't see any smaller ones on their site.
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  • Adam_MDAdam_MD Frets: 3420
    edited January 2018
    Shoot me a pm with your address and I’ll send you some or these are the ones you want from bitsbox to fit in an enclosure

    https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=65_76_77&products_id=361
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  • PhiltrePhiltre Frets: 4164
    @Adam_MD Right, you did mention those ones in your earlier post in this thread. I'll get a couple in my next order, cheaper too. BTW - thanks for the offer but I may as well stick them on my next Bitsbox order as I have to get the proper pots, enclosure and some vero as well.

    One interesting general observation - I was surprised that my trial breadboard build wasn't noisy at all. I was expecting that with all those wires and connections the circuit might have picked up noise. But it's very clean (for an overdrive, that is).

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